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Bay Area waterfront communities and airports could face chronic flooding from rising seas if carbon emissions continue to rise, according to the nonprofit group Union of Concerned Scientists. (City of San Jose)

Chronic flooding from rising seas could plague many Bay Area waterfront communities such as East Palo Alto, Alameda and San Mateo within four decades, a nonprofit science group said in a report released Wednesday.

While other studies have predicted inundation of coastal cities, this new study by the Union of Concerned Scientists is the first to put dates on when towns that ring the San Francisco Bay would regularly experience chronic flooding.

Rather than slam shoreline communities with epic floods every few years, rising sea levels threatens to flood streets, yards, parks, homes and businesses in low-lying areas several times a year, the scientists said

“Cities around the San Francisco Bay will begin to experience more frequent and disruptive flooding in the coming decades and will have to make tough decisions around whether to defend existing homes and businesses or to retreat,” said Erika Spanger-Siegfried, senior analyst in the Climate and Energy Program at UCS and a report author.

Airports and low-income housing in low areas are particularly vulnerable, the study said.

While airports can draw on business income to pay for defenses against rising seas, many poorer neighborhoods are hard pressed to afford bigger seawalls or levees or to move people out of flood-prone areas, said Kristy Dahl, a UCS climate scientist and co-author of the report.

She said the report underscores the need for federal policies to help local communities.

“We shouldn’t have some communities left behind simply because they don’t have the resources of their neighbors,” Dahl said in an Oakland press conference to discuss the study. “A large number of these communities don’t have the resources they truly need to adapt.”

Last year, the federal government announced its first grant to buy and relocate a small town — Isle de Jean Charles, La. — for $48 million after concluding it was not worth trying to save the community in place.

The Union for Concerned Scientists study assessed three scenarios — low, intermediate and high sea-level rise — by the years 2060 and 2100, depending on the pace of emissions and melting rates of polar ice. An interactive series of maps shows when inundated communities may reach tipping point, with at least 10 percent of usable land flooded at least 26 times per year.

The study found that:

● By 2060, in the high sea level rise scenario, parts of many Bay Area communities would face flooding 26 times or more per year, or every other week. Communities with affected neighborhoods include Alameda, Oakland, Palo Alto, East Palo Alto, San Mateo, Burlingame, San Francisco, Corte Madera and Larkspur.

● By 2100, in the intermediate sea level rise scenario, chronic flooding would affect public infrastructure such as San Francisco International Airport, Oakland International Airport, San Quentin State Prison, Moffett Federal Airfield and the Bay Bridge.

● By 2100, in the intermediate sea level rise scenario, two Bay Area communities would see more than 10 percent of their land chronically flooded: Alameda and San Mateo.

● By 2100, in the high sea level rise scenario, more than half of Alameda, about 11 percent of South San Francisco and about 14 percent of Oakland’s land area would be chronically flooded.

“Imagine what it would be like to have your driveway and backyard flooded every every other week on average,” Dahl said, “And you can’t let your kids play in the back yard because it’s flooded.”

The “low scenario” assumes a San Francisco Bay water level rise of around 2 feet by 2100, a carbon emissions decline, and global warming limited to less than two degrees Celsius — in line with the primary goal of the Paris Agreement.

The “intermediate scenario” projects a four-foot water level rise and carbon emissions peaking around mid-century and about four feet of sea level rise globally. In the high scenario, emissions rise through the end of the century and ice melts faster, causing 6.5 feet of sea level rise.

The group applauded efforts by cities such as San Francisco and Foster City, which already have begun planning where and how to build seawalls and levees. Other regions — such as the cities of Alameda, Hayward and Oakland and Contra Costa, San Mateo, Alameda and Santa Clara counties — are close behind, identifying potential strategies.

Welcoming the report, David Lewis of the Oakland-based nonprofit Save The Bay said it underscored the need for the Federal Emergency Management Agency to update Bay Area flood maps to reflect new projections. Those flood maps determine where property owners can and cannot qualify for federally-subsidized flood insurance, and where communities must construct additional flood protection to retain that insurance.

He urged the state to press FEMA to update the maps. Congress also must be prodded to provide funding for the updates, he added.

“If maps don’t incorporate projections for sea level rise — and for increased frequency of flooding from extreme storms independent of sea level rise — then communities have a false sense of security, and property values, as well as public and private planning and development decisions, don’t accurately reflect risks,” said Lewis.

“Ask those homeowners near Coyote Creek,” which flooded last winter, he said.

Lisa M. Krieger is a science writer at The Mercury News, covering research, scientific policy and environmental news from Stanford University, the University of California, NASA-Ames, U.S. Geological Survey and other Bay Area-based research facilities. Lisa also contributes to the Videography team. She graduated from Duke University with a degree in biology. Outside of work, she enjoys photography, backpacking, swimming and bird-watching.

Until a few years ago, marine scientists studied the ocean mostly through sight. Now, with Monterey Bay Aquarium Research Institute’s deployment of an underwater microphone called a hydrophone, scientists are studying the ocean through sound.